Overcoming the limitation of in-situ microstructural control in laser additive manufactured Ti-6Al-4V alloy to enhanced mechanical performance by integration of synchronous induction heating

doi:10.1016/j.jmst.2021.02.069

Abstract

Abstract:

Although a variety of processing routes were developed to in-situ manipulate microstructure for fabricating high-performance Ti-6Al-4V alloy by directed energy deposition (DED), the in-situ microstructural control ability has been limited and lead to a narrowed mechanical property control range. This work proved the microstructural correlation between β-grains and α-laths resulting from the unique thermal characteristics of DED for the first time and solved such a dilemma through synchronous induction heating assisted laser deposition (SILD) technology. The results confirmed that the laser energy and inductive energy have a different effect on the solidification and solid phase transformation conditions. By adjusting the laser-induction parameters, the microstructural correlation can be tuned; the β-grains and α-laths can be controlled relatively separately, thereby significantly enhancing the ductility of as-deposited sample (elongation from 14.2% to 20.1%). Furthermore, the mechanical properties of the tuned microstructures are even comparable to that of DED Ti-6Al-4V with post heat treatment, which indicates that the potential of SILD to be a one-step manufacturing process to fabricate high performance components without post heat treatment. Furthermore, the tensile testing results of the tuned microstructures indicate that α-lath size is more influential on the mechanical properties than the β-grain size due to its stronger hindering effect on the slipping of dislocations. This work promotes the understanding of the microstructural formation mechanism in DED titanium alloy and proves that the combination of synchronous induction and laser can expand the ability to control the microstructure and properties of multi-layer deposition.

Key words: Directed energy deposition, Additive manufacturing, Ti-6Al-4V, In-situ microstructural control, Mechanical properties

Wei Fan, Hua Tan, Fengying Zhang, Zhe Feng, Yongxia Wang, Lai-Chang Zhang, Xin Lin, Weidong Huang. Overcoming the limitation of in-situ microstructural control in laser additive manufactured Ti-6Al-4V alloy to enhanced mechanical performance by integration of synchronous induction heating[J]. J. Mater. Sci. Technol., 2021, 94: 32-46.

Figures/Tables 20

Fig. 1. In-situ microstructure control limitation in DED Ti-6Al-4V and its solution. (a) relationship between the processing parameters and relative (a) β-grain size, G-1/2RS-1/4 and (b) α-lath size, thold0.33RC-0.31; (c) linear relationship between G-1/2RS-1/4 and thold0.33RC-0.31; (d) comparison of the relationship between Wβ and Wα in DED Ti-6Al-4V [9, 11, 15, [29], [30], [31], [32], [33], [34], [35]].

Table 1 Chemical composition of the Ti-6Al-4V alloy powder used for DED process.

Element	Al	V	Fe	C	N	H	O	Ti
Content (wt. %)	6.15	4.00	0.30	0.10	0.05	0.02	0.20	Balanced

Fig. 2. Schematic diagrams of the experimental procedure: (a) deposition pattern of synchronous induction heating assisted laser deposition (SILD) process; (b) Ti-6Al-4V thin wall sample deposited by SILD; (c) preparation of the tensile test specimens and metallographic observation specimen; (d) dimensions of tensile specimens.

Table 2 DED processing parameters used for synchronous induction assisted laser deposition experiments.

Sample	Input current (induction heater), I	Laser power, P	Scanning speed, v	Feed rate of powder, r_p	Beam diameter, D	Layer thickness, ΔZ
0-1000	0 A	1000 W	400 mm/min	10 g/min	3 mm	0.5 mm
100-1000	100 A	1000 W
200-1000	200 A	1000 W
200-850	200 A	850 W
200-700	200 A	700 W

Table 3 Thermo-physical properties of Ti-6Al-4V alloy for calculation [39]

Temperature, T (°C)	Density, ρ (kg/m³)	Thermal conductivity, λ (W/(m°C))	Heat capacity, c (J/(kg°C))
20	4420	7.0	546
205	4395	8.8	574
500	4350	12.6	651
995	4282	22.7	753
1100	4267	19.3	641
1200	4252	21.0	660
1600	4198	25.8	732
1650	3886	83.5	831
2000	3818	83.5	831

Fig. 3. Grain evolution of the SILDed Ti-6Al-4V: Optical microstructures of (a) 0-1000, (b) 100-1000, (c) 200-1000, (d) 200-850, and (e) 200-700 samples; the statistical results of the grain width and evolution trend with (f) input current and (g) laser power.

Fig. 4. The α phases in the prior β-grain under different SILD processing parameters: SEM images of (a) 0-1000, (b) 100-1000, (c) 200-1000, (d) 200-850, and (e) 200-700 samples.

Fig. 5. Statistical results of the fraction, width and aspect ratio of α-laths under different SILD parameters: (a) fraction with different input current; (b) fraction with different laser power; (c) width with different input current; (d) width with different laser power; (e) aspect ratio with different input current; (f) aspect ratio with different laser power.

Fig. 6. Relationship between the β-grain width (Wβ) and α-lath width (Wα) in SILD Ti-6Al-4V: the influence of (a) input current and (b) laser power on the β-grains and α-laths, evaluated by the value of Wβ/Wα; (c) comparison of the relationship between Wβ and Wα in DED and SILD.

Table 4 Parameters used in simulation [22]

Parameters	values
Laser absorptivity of Ti-6Al-4V, δ	0.3
Heat convection coefficient of the substrate, h_sub	100 W/(m²°C)
Heat convection coefficient of the deposition, h_dep	7 W/(m²°C)
Heat emissivity coefficient of the deposition, ε	0.7
Ambient temperature of the substrate, T_ini	25°C
Heat intensity of induction heating, Q_I	315 W/(mm³ A)
Substrate size, S_s	72 mm × 3 mm × 50 mm
Mesh size, S_m	3 mm × 3 mm × 0.5 mm

Fig. 7. Calculated temperature field during the 20th layer deposition process under different laser-induction parameters: (a) when the laser scans across the middle point of the 20th layer; (b) 10 s cooling after the 20th layer deposition process.

Fig. 8. Calculated thermal cycles at middle point of the 20th layer under different SILD parameters and corresponding temperature distributions when laser scans across the middle point of the 20th layer with different: (a) input current and (b) laser power.

Fig. 9. Solidification conditions and solid-phase transformation conditions under different SILD parameters: (a) the temperature gradient in front of solid-liquid interface and solidification rate; (b) the cooling rate at Tβ and hold time in the two-phase region; (c) the influence of input current on the ratio of relative Wβ / relative Wα; (d) the influence of laser power on the ratio of relative Wβ / relative Wα.

Fig. 10. Elemental maps of vanadium in the samples under different SILD parameters.

Fig. 11. XRD patterns of the DED Ti-Al-V samples under different SILD parameters: (a) XRD profiles and (b) enlarged profiles of α(001) peak.

Table 5 Vickers hardness test results for SILD samples.

Sample label	0-1000	100-1000	200-1000	200-850	200-700
Hardness (Hv)	385±15	360±8	340±5	345±5	367±8

Fig. 12. Tensile test results for samples under different laser-induction parameters: (a) engineering stress vs. strain; (b) comparison of mechanical properties for Ti-6Al-4V fabricated by DED, DED+HT and SILD [5, 11, 35, [43], [44], [45], [46], [47], [48], [49]].

Fig. 13. Fractography of samples under different laser-induction parameters: (a) 0-1000; (b) 100-1000; (c) 200-1000; (d) 200-850; (f) 200-700.

Fig. 14. Relationship between microstructure size and mechanical properties: (a) the influence of Wβ on Vickers hardness; (b) the influence of Wα on Vickers hardness; (c) the influence of Wβ on yield strength; (d) the influence of Wα on yield strength; YS data fitted by Hall-Petch relationship with (e) Wβ and with (f) Wα.

Fig. 15. EBSD and SEM results of deformed Ti-6Al-4V sample fabricated by SILD: (a) the band contrast map; (b) IPF map; (c) kernel average misorientation map; (d) schematic diagram of the dislocation slip; (e) micrograph of the longitudinal section close to the fracture surface. The results indicate that the dislocations are preferred to pile up at the α/β interfaces thereby resulting in voids and cracks.

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